Control system



April 23, 1935.

CONTROL SYSTEM FiledOct. 15, 1952 0/ 5 I Invent OT:

Albert |-l. lvnttag, by

H is Attowflweg.

A. H. MITTAG 21,998,938

Patented Apr 2 3, 1935 UNITED STATES CONTROL SYSTEM Albert H. Mittag, Schenectady, N. Y., assignor to General Electric Company,

New York a. corporation of Application October 15, 1932, Serial No. 637,971 9 Claims. 01. 250-27 This invention relates to control systems, more particularly to systems in which a translating circuit is supplied from electric valve apparatus, andan object of the invention is the provision of a simple reliable, efficient and improved system of this character.

More specifically the invention relates to control systems in which at least two electric valves are utilized to supply current in opposite directions to a translating circuit. In systems of this character, voltage is supplied to grids of the valves in such a'mann'er that a positive voltage is applied to the grid of one valve so as to actuate that valve to supply current to the translating circuit in one direction and a negative voltageis applied to the second valve to render the second valve inactive; and means are provided for reversing the polarities of the grid voltages thereby deenergizing the active valve and energizing the inactive valve to supply current to the translating circuit in the reverse direction. Within certain limits an electric valve is non-conducting when a negative voltage isapplied to the grid. It has been determined, however, that if the negative voltage on the grid of the valve is increased beyond a. predetermined critical value, the valve will break flown and become conducting. This is especially true of valvesof the vapor electric type. It has happened that withvalves connected to supply current to a circuit in opposite directions, an increase in the positive grid voltage of the active valve to increase the current supplied to the load circuit resulted inincreasing the negative voltage of the inactive valve beyond this critical value, thereby causing the inactive valve to break down and'become conducting, with the result that both valves supplied currents to the' circuit in the same instant which interfered with the proper operation of the system. Accordingly a more specific object of this invention is the provision of an improved system in which this disadvantage is eliminated.

In carrying the invention into effect in one form thereof, at least provided for supplying current in respectively opposite directions to the load circuit and means are provided for supplying voltage to the grids of these valves in such a manner that a positive voltage is applied to the grid of one valve to energize that valve and a negative voltage is an plied to the grid of the second-valve to render the second valve inactive togetherwith means for limiting the negative grid voltage of the inactive valve to a predetermined value to prevent eneropposite directions at two electric valves are' gization of the inactive valve." In one form of theinvention a discharge device such as an elec-. tric glow lamp is connected between the cathodes and grids of each of the valves. In another form of the invention, a non-linear resistance is connected between the cathodes and grids.

In illustrating the invention in one form thereof it is shown as embodied in a follow-up system, in which a load body is driven into positional agreement with a pilot device by means of an electric motor supplied from electric valve apparatus.

For a better and more complete understanding of the invention reference should now be hadto the following specification and to the accompanying drawing in which Fig. l is a simple, diagrammatic illustration of an embodiment of the invention and Fig. 2 is a vector diagram of certain electrical operating characteristics of the system of Fig. l facilitating the understanding of the invention.

Referring now to the drawing a suitable translating circuit including the direct current-elem tric motor I0 is supplied with direct current in respectively opposite directions by means of the electric valve apparatus II to the output circuit of which the armature terminals of the motor H) are connected by means of the conductors i2 and I3. The electric valve apparatus H is itself supplied from any suitable source such for example as the three phase supply source represented by the three supply lines M by means of a suitable supply transformer I 5, the terminals of the primary'winding of which are connected to the middle and. lower supply line M as illustrated. As shown the electric valve apparatus ll comprises a pair of electric valves I6, I! connected for full wave rectification for supplying direct current in one direction to the armature of the electric motor l0 and a similar pair of electric valves l8, l9 also connected for full wave rectifica- 'tion to supply direct current to the armature of the motor I0 in the reverse direction.

The electric valves l6, l1, l8 and I9 are prefer-' ably of the three electrode type and as shown the valves l6, l1 are respectively provided with cathodes 20 and 2|, grids 22 and23, and plates or anodes 24 and 25. The pair of valves 3 and I 9 have similar elements as illustrated. These valves are preferably of the vapor electric type,

'i. e., after exhaust the envelopes are filled with a suitable inert: gas such for example as mercury vapor,-the presence of which within the envelope ,servesto convert the usual pure electron discharge into an'arc stream thus constituting the valves electrostatically or and controlled arc rectifiers. a

The plates 24, of the valves l6 and. I! are respectively connected to the opposite terminals of the secondary winding portion l5 or the transformer IS, the mid-point of which winding portion is connected to the conductor l3 through a suitable smoothing reactance 26 and a current limiting resistance 21 connected in series relationship with each other and the cathodes 20 and 2| of this pair of electric valves are directly con-. nected to the conductor i2. Current flows in the plate circuit of the electric valves only during that half cycle of the voltage applied to the anode in which the potential of the anode is positive with respect to that of the cathode and this flow of current is initiated by supplying to the control grids a voltage that is more positive with respect to'the anode voltage than a predetermined critical value. By varying the point in the cycle of the anode voltage at which the grid voltage becomes sufliciently positive to initiate current flow in the anodecircuit, the average value of the current flowing in the latter circuit is controlled as desired and it will thus be seen that if an alternating voltage is supplied to the control grid of the electric valve apparatus, the

point in the cycle of applied anode voltage at which the grid becomes sufliciently positive to initiate current flow in the anode circuit may be varied by shifting the phase of the grid voltage with respect to that of the anode voltage.

Asshown in the drawing, alternating voltage'is supplied to the grids 22, 23 of the electric valves l6 and I1 by means of a suitable grid transformer 28, the opposite terminals ofthe secondary winding of which are respectively connected to the grids 22 and 23 by means oi conductors 29 and 30 and alternating voltage is likewise supplied to the grids of the electric valves l8 and H! by means of the grid transformer 3|. The primary windings of the grid transformers 28 and 3! are connected in series relationship with each other and are in turn connected to the terminals of the secondary winding of a grid bias transformer 32, one terminal of the primary winding of which is connected to the upper supply line H and the opposite terminal of which is connected to an intermediate point on the primary winding of the power supply transformer l5.

The connection of one terminal of the primary winding of the grid supply transformer 32 to an intermediate point of the power supply transformer l5 serves to shift the'phase of the grid bias voltage with respect to that of the anode voltage as illustrated in the vector diagram of Fig. 2 in which the vector E, represents the voltage supplied to the anodes and the vector Eh, represents the grid bias voltage supplied to the grids. The phase relationship of the voltages applied to the grids and anodes of the electric valves is illustrated by the angle between the vectors Ea and Es. When the grid voltage is in phase with the anodevoltage, maximum current is delivered in the output circuit of the electric valve and whenthe grid voltage is out of phase with the anode voltage, substantially no current flows in the output circuit. For intermediate phase relationships of the grid and anode voltage, the

I current flow in the output circuit is proportionate to the phase relationship. The phase relationship between the grid and anode voltages is controlled by any suitable means such for example as the electrical motion transmitting device 33 and a receiving device 34 connected between the voltage to the grids of the electric valve apparatus. The transmitting device vhas a stator member provided with a polycircuit stator winding 35 which'is physically similar to the three phase winding of an alternating current dynamo elec tric machine and is also provided with a rotor member having a single phase rotor winding 36 arranged in inductive relationship with the stator winding 35. a

The receiving device 34 is provided with a stator winding 31 and a singlephase rotor winding 38 which are in all respects identical with the corresponding windings of the transmitting device 33. As shown three equi-spaced stator winding terminals of the transmitting device 33 are connected to three corresponding stator winding terminals of the receiving device 34 by means of the three conductors 39. The alternating voltage is supplied to the rotor winding 35 of. the transmitting device from the source M to the'middle and lower supply lines of which the winding 36 is connected by means of conductors 40. The current flowing in the rotor winding 36 sets up an alternating magnetic field, the position of which in space with respect to the stator winding 35 depends upon the position of the rotor winding 36 with respect to a predetermined axis of the stator winding 35. This alternating magnetic field induces alternating voltages in the circuits of stator winding 35 which in turn estab-- lish current flow in the stator winding 31 of the receiving device 34. The currents flowing in the circuits of the stator winding 31 reproduce the magnetic field of the transmitting device with the result that a voltage is induced in the rotor winding 38 of the'receiving device, the magnitude of which depends upon the relative position of an axis of the rotor winding with respect to the axis of the magnetic field of the stator winding. The voltage thus induced in the rotor winding 38 of the receiving deviceis supplied to the grid circuit of the electric valve apparatus H by means of a step-up transformer 4|, the terminals of the secondary winding of which are respectively connected to the rotor winding 38 by means of conductors 42 and the opposite terminals of the secondary winding of which are respectively connected to a mid-point of the secondary. winding of the grid bias transformer 32 and to a point between the primary windings of the individual grid transformers 28 and 3|.

Since the rotor winding 36 of the transmitting device is connected to the same phase of the supply source as that from which the anodes of the electric valve apparatus are supplied the component of voltage supplied to the grid circuit of the electric valve apparatus from the receiving device 34 is substantially in phase with the anode voltage as represented in Fig. 2 of the drawing by the vector Ec for the pair of tubes l6, H; the vector Ec shown substantially in phase with the vector Ea representing the anode voltage. The corresponding component voltage for the other pair of valves is represented by the vector Eel, substantially out of phase with the vector Ec. It will thus be seen that the resultant voltage supplied to the grids of'the electric valve apparatus is made up of the two components grid bias transformer 32.

anode voltages are substantially out of phase of correspondence in. one direction. If the system is out of correspondence in the opposite direction, the vectors E0 and E01 and the voltages which they represent are reversed. When the axis of the rotor winding 38 is at right angles with the axis of the magnetic field of the stator winding 31 of the receiving device, the voltage induced inthe rotor winding 38is substantially zero and consequently the component of voltage supplied to the grid of the electric valve apparatus from therotor winding 38 and represented by the vectors Ec and En is substantially zero, as a result of which the vectors E and E 1 coincide with the vector Eb; that is to say, the voltage actually applied to the grid is the grid bias voltage supplied from the source I4 through the When the grid and as represented by the phase relationship of the vectors Ea and Eb the current flow in the output circuit .of the electric valve apparatus is minimum or substantially zero. When the axis of the rotor winding 38 is substantially parallel with the axis of the magnetic field of the stator winding 31 of the receiving device, the voltage induced in the rotor winding 38 is maximum with the result that the magnitude of the vectors EC and E01 is correspondingly increased. As the magnitude of vectors EC and E01 increases, the vector Eg which represents the actual or resultant voltage applied to the grid is shifted closer to the in-phase relavalves l6, l1 represented by the vector Ea and the flow of current in the output circuit of the electric valve apparatus is correspondingly increased. For the other pair of valves the vector E 1 is shifted further out of phase with the anode voltage.

It will be clear that when the polarity of the voltage represented by the vector Ec applied to the grids 22 and 23 of the electric valves L6 and I1 is positive the polarity of the voltage 'applied to the grids of the electric valves I8, l9 will be negative, with the result that the valves l6, l1 supply current to the translating circuit I0, l2, IS in a predetermined direction and the valves I8, H! are inactive. For the purpose of providing a high degree of accuracy in the follow-up system the transformer 44 is so dewith respect'to the anode voltage Ea'so one pair of electric valves will become active and supply current in the output circuit to the driving motor l0. Consequently when the rotor of the receiving device 34 is out of correspondence with the'rotor of the transmitting device 33 by a large amount, the component of voltage supplied to the grid circuit from the rotor winding 38 is correspondingly increased. The magnitude of this voltage is increased to such an extent that the negative voltage applied to the grid of the inactive pair of electric valves exceeds the predetermined critical value which. causes this pair of tubes to break down and become conducting with the result that both pairs of tubes attempt to "supply, current to the translating circuit in opposite directions at the same time, thereby interfering with the proper operation of the system. i

anode.

As far as is now known, this undesirable condition may be due to either, or a combination of two phenomena; one known as grid emission and the other known as cold cathode discharge. In explaining the phenomenon of grid emission it is pointed out that the cathodes 20, 2|, etc. of the valves are usually coated with an electron emitting oxide. During the usual operation of the valve some of this oxide may be and often is deposited on the grid by the arc stream. When the control object 33 and the controlled object 34 become out of correspondence by a large amount, it is possible, in the system illustrated, for the grid to become as much as 1100 volts negative with respect to the cathode which in turn may at the same time become as much as 1200 volts negative with respect to the anode. In other words, the grid may'become as much as 2300 volts negative with respect to the anode. This voltage, together with'the oxide deposited on the grid, causes the grid to become a cathode and initiate a discharge between itself and the anode, which discharge though small is suflicient to ionize the space between the cathode and anode and initiate the discharge between the cathode-and anode.

The phenomenon of cold cathode discharge takes place between the grid and cathode. It

become as much as 1200 volts negative with respect to the cathode, or vice versa, the cathode become 1200 volts positive with respect to the grid, This causes the cathode to become an anode and the grid to become a cold cathode thereby initiating ,a discharge between the two which is sufficient to ionize the tube and to start the regular discharge between the cathode and The above conditions are often further aggravated by a grid structure having many sharp points.

In order to limit the magnitude of the negative voltage applied to the grids of the electric valves I6, l1, l8 and IE to a permissible value,

electric glow. discharge lamps 43, 44, 45 and 46 are respectively connected between the grids and cathodes of corresponding electric valves l6, l1, l8 and I9, and suitable resistance devices 41, 4B, 49 and 50 are respectively includedin the connection in series relationship with the corresponding electric discharge lamps 43, 44, 45 and 46, also resistances 52 and 53 are ineluded in circuit between the terminals of the secondary winding of the grid transformer 28 and.the. grids 22 and 23 and similar resistances 54 and 55 are included in circuit between the secondary of transformer 3| and the grids of valves l8 and IS. A glow discharge lamp comprises a pair of spaced electrodes within an envelope which is filled with a suitable gas such forexample as neon. As long as the voltage, applied to the electrodes of a glow lamp, is below a certain critical value no discharge takes place between the electrodes whereas when this critical value is reached the gas is ionized and a discharge takes place across thespace between the electrodes.

Instead of the glow lamp illustrated in the drawing, a'suitable non-linear resistance device may be employed and connected between the grids and cathodes of the electric valve. A nonlinear resistance is a resistance, the conductance of which varies with some power of the impressed voltage other than unity. Althoughany suitable non-linear resistance may be employed it is preferred to utilize a resistance composed of silicon carbide crystals held together by a suitable binder such for example as disclosed in U. S. Patent No. 1,822,742+-McEachr0n, dated September 8,-

1931. When a low voltage is impressed upon a non-linear resistance of this character, a very negligible current flows through the resistance. However, when the impressed voltage is increased beyond a predetermined critical value, the cur-- rent increases rapidly. Thus when non-linear resistances of this character are utilized in placeof the glow lamps the resultant efiectin the circuit is substantially the same; that is to say, when the voltage is below a certain critical value substantially no current flows through the resistance and when the impressed voltage exceeds this critical value avery large current flows through the resistance.

With the above understanding of the apparatus and its organization in the completed system the operation of the'system itself will readily be understood from the detalled description whi h follows:

Assuming the rotors of the transmitting and receiving devices 33 and 34 to be in exact correspondence, the axis of the rotor winding 38 of the receiving device is at right angles with the magnetic held of the stator winding and no voltage is induced in the rotor winding 38 as a result of which the phase relationship between the voltages applied to the grids and anodes of the electric valve apparatus is as represented by the vectors Ea and Eb, and both pairs of electric valves l6, l1 and l3, l9 are non-conducting and'the system is at rest. If the rotor of the transmitting device 33 is rotated the magnetic tionship of the resultant grid voltage with respect to the anode voltage represented by the vector Ea. Thus one pair of electric valves becomes conducting and supplies current to-the electric motor In in such a direction as to cause the motor in to drive the rotorof the transmitting device in a direction to reestablish correspondence between the rotors of the transmitting and receiving devices. 1 As the'phase oi the grid. voltage is advanced with respect to the anode 'voltages of the valves l6, II it is retarded with respect to the anode voltages of the valves I3, IS with the result that this pair of valvesremains inactive due to the factxthat when the grid voltage of one of the valves IQ, ll is positive the grid voltage oi the corresponding valve l3 or I9 is negative.

er 4|, with the result that the magnitude of the negative voltage supplied to the grids of the valves |8, |9 exceeds the critical value at which the valves l8 and I9 would become conducting and attempt to supply current to the motor ill in a direction opposite to that in which current is being supplied by the pair of electric valves l6, l1. However, the critical value of breakdown voltage of the electric glow lamps 45, 46 is reached before the critical value of negative voltage is prevented from exceeding the critical value at which the valves l8 and I9 become conducting.

If the impedance in transmitting device 33, receiving device 34 and transformer 4| is of a magnitude sufficient to cause a large enough drop in the voltage output of transformer 4| when the additional load taken by the glow lamps is added. so as not to cause the negative grid voltage to exceed the critical value, the resistances.

52, 53, 5'4 and may beomitted.

Rotation of the rotor of the transmitting device 33 in the oppositedirectlon fromthat as-' sumed in the above discussion results in applying positive voltage to-the grids of the valves I8 and I9 so that these valves become conducting and supply current to the motor Ill in a direction such that the motor l0 rotates the rotor member of the receiving device 34 in a direction to reestablish correspondence, and a negative voltage is applied to the grids 22, 23 of the electric valves l6 and I1 soas to' maintain this pair of valves in a deenergized condition. If the rotors of the transmitting and receiving devices are out of correspondence by alarge amount the electric glow lamps 43 and 44 prevent-the negative voltages of grids 22 and 23 from exceeding the critical value at which the valves "5 and I1 would be conducting in exactly the same manner as that described in connection with the pair of valves l8 and I9.

When non-linear resistances are utilizedin placeof the electric glow lamps 43, M, 45 and 46 the operation to prevent excessive negative grid voltage is exactly the same as that described in connection with the use of, the glow lamps.

Although in accordance with the provisions 0 the patent statutes, this invention is described in concrete form, 'it will be understood that the apparatus and connections described and shown in the drawing are merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of the invention or the scope of the annexed claims.

What I claim as newand desire to secure by Letters Patent of the United States, is:

1. A control system comprising a translating circuit, supply means for said circuit comprising a plurality of electric valves each provided with a cathode and a control grid, means for supplying a'positive voltage to the grid of one of said valves anda negative voltage to the grid of an other of said valves, and means for limiting said negative voltage to a predetermined value.

2. A control system comprising a'translating circuit, supply means for said circuit comprising at least two electric valves each having a cathode and a control grid and connected to supply current to said circuit in respectively opposite di-, rections, means for supplying a positive voltage to the grid of one of said valves to render said valve active and a negative voltage to the grid directions, means for said alternating voltage, and a tive valve.

3. A control system comprising a translating circuit, supply means for said circuit comprising i at least two electric valves, each provided with a cathode and a control grid and connected to supply current to said circuit in respectively opposite directions, means for supplying an alternating voltage to said grids so that an instantaneous positive voltage is applied to the grid of one of said valves to energize said valve and an instantaneous negative voltage is applied to the grid of a second valve to render said second valve inactive, and a device connected between the grid and cathode of each of said valves for preventing said negative voltage attaining a value suflicient to energize. said inactive valve.

4. A control system comprisinga translating circuit, supply means for said circuit comprising at least two electric valves each provided with a cathode and a control grid and connected to supply current to said circuit in respectively opposite directions, means for supplying an alternating voltage to said grids so that a positive voltage is applied to the grid of one of said valves to energize said valve and a negative voltage to a second of said valves to render said second valve inactive, means for reversing the polarity of said voltages and a device connected between the cathode and grid of each of said valves for limiting said negative voltage to a predetermined value and thereby prevent said negative voltage from v energizing the inactive valve. a

5. A control system comprising a translating circuit, supply means for said circuit comprising at least two electric valves each having a cathode and a control grid and connected for supplying current to said circuit in respectively opposite supplying an alternating voltage to said grids so that a positive voltage is applied to the grid of one of said valves to energize said valve and a negative voltage is applied to the grid of a second of said valves to render said valve deenergized, means for varying device connected between the cathode and grid of said valves for limiting said negative voltage to a predetermined value to prevent energization of said second valve.

6. A control system comprising a load circuit, supply means for said circuit comprising at least two vapor electric valves each having a cathode and a control grid and connected to said circuit for supplying current thereto in respectively opposite directions, means for supplying a positive voltage to the grid of one oi said valves to energize said valve and a negative voltage to the grid of a second valve to render said second'valve inactive, and an electric glow discharge device connected between the grid and cathode of each of said valves for limiting said negative voltage to a predetermined value to prevent energization of the inactive valve. o

7. A control system comprising a load circuit,

supply means for said circuit comprising at least twovvapor electric valves each provided with a cathode and a control grid and connected for supplying current to said circuit in respectively opposite directions, means for supplying an alternating voltage to said grids so that a positive voltageis applied to the grid of one of said valves to energize said valve and a negative voltage is applied to the grid of a second valve to maintain said second valve inactive, means for reversing the polarity of the voltages applied to said grids so as to energize the inactive valve and deenergize the active valve; means for increasing said alternating voltage, and an electric glow discharge device connected between the grid and cathode of each of said valves for limiting the negative voltage to a predetermined value to prevent energization of the inactive valve.

8. A control system comprising a load circuit, supply means for said circuit comprising at least two electric valves each provided with a cathode and a control grid and connected to said circuit so as to supply current thereto in respectively opposite directions, means for supplying a voltage to the grids of said valves so that a positive voltage is applied to the grid of one valve to energize said valve and a negativevoltage is applied to the grid of a second valve to render said second valve inactive and a non-linear resistance device connected between the cathode and grid of each of a predetermined value to of the inactive valve.

9. A control system comprising a load circuit, supply means for said circuit comprising at least two vapor electric valves each having a cathode and a control grid connected to supply current to said'circuit in respectively opposite directfons, means for supplying an alternating voltage to said grids so that a positive voltage is applied to the grid of one of said valves to energize said valve and a negative voltage is applied to the grid of a second valve to render said second valve inactive, means for reversing the polarities of said prevent energization grid voltages to energize the inactive valve and 

